This invention relates to a conversion coating composition for magnesium and magnesium alloy articles that achieves similar results to a chromate conversion coating, without the hazardous effects of chromium. In addition, the invention relates to a method of applying the conversion coating composition to magnesium and magnesium alloy articles before painting to prevent corrosion.
The invention relates to a conversion coating for preparing magnesium and magnesium alloy parts prior to painting. Paint adhesion to magnesium and magnesium alloy substrates is poor if the substrate are not first coated with a conversion coating. Paint does not bond well to the natural oxide of magnesium, and the rapid oxidation of magnesium makes it impractical to clean and deoxidize the surface of the article prior to painting. Consequently, painted magnesium that is commercially manufactured is coated with a conversion coating prior to painting.
Several methods are commonly used as conversion coatings to prepare magnesium and magnesium alloy articles prior to painting, including chrome bearing conversion coatings and electrolytic anodizing. Both chrome bearing conversion coatings and electrolytic anodizing are well known in the art and have been the subject of numerous patents.
Painted magnesium parts are also susceptible to peeling in corrosive environments. Corrosion proceeds laterally under the surface of the painted magnesium, typically starting at a scratched area, until the paint either forms a blister or peels away. Coating with a corrosion inhibitor before painting prevents the paint from peeling.
The conversion coating of the present invention, provides an adherent and corrosion resistant base on magnesium and magnesium alloy substrates in preparation for painting.
The composition of the present invention achieves similar or better results than chromate conversion coatings without the use of chromium. Chromium is extremely toxic even at low levels and is an increasingly regulated material. It is therefore beneficial to use a product that does not contain chromium. In addition, the method of the present invention is an immersion process, so racking and external power, such as is necessary in anodizing operations, are not needed, providing a cost and product efficiency benefit over anodizing.
The inventors herein have discovered a novel composition and method for creating a conversion coating on magnesium. The invention comprises contacting magnesium or magnesium alloy with a composition comprising:
1) A source of vanadate ions;
2) A material comprising phosphorus selected from the group consisting of sources of phosphite ions, sources of hypophosphite ions, sources of phosphate ions, sources of phosphorus ions, sources of hypophosphorus ions, and combinations of the foregoing;
3) Nitric acid or a source of nitrate ions;
4) Optionally, but preferably, boric acid or a source of borate ions; and
5) Optionally, but preferably, a source of fluoride ions or fluoroborate ions.
The composition for use in the process of the present invention creates a unique conversion coating on magnesium and/or magnesium alloys. This conversion coating inhibits the subsequent corrosion of the treated surfaces and increases the adhesion of subsequent coatings such as paints, lacquers, and other such finishes to the treated surfaces. These and other advantages can be achieved by treating the surfaces of magnesium or magnesium alloys with a composition comprising:
1) A source of vanadate ions;
2) A material comprising phosphorus selected from the group consisting of sources of phosphite ions, sources of hypophosphite ions, sources of phosphate ions, sources of phosphorus ions, sources of hypophosphorus ions, and combinations of the foregoing;
3) Nitric acid or a source of nitrate ions;
4) Optionally, but preferably, boric acid or a source of borate ions; and
5) Optionally, but preferably, a source of fluoride ions or fluoroborate ions.
Vanadate is added to the composition as any corresponding soluble salt or acid of vanadium. Some examples include sodium vanadate, potassium vanadate, and ammonium vanadate. Ammonium vanadate is preferred, preferably at a concentration of about 5 grams/liter. The concentration of vanadate in the mixture should preferably be in the range of 0.1 to 5 grams per liter, where the upper concentration is limited by the solubility of the vanadate in the mixture.
The concentrate of nitric acid or nitrate ions in the solution may range from 1 g/l to near saturation but preferably is from about 25 g/l to about 200 g/l. If nitric acid is used, then it must be neutralized so that the pH of the solution preferably ranges from about 1 to about 4. Neutralization is preferably carried out with ammonium hydroxide. In the alternative, sources of nitrate such as sodium nitrate, potassium nitrate, or ammonium nitrate may be utilized with ammonium nitrate being preferred.
The phosphorus comprising material can be any of a variety of phosphorus comprising materials including hypophosphorus acid, phosphorus acid, sodium (or potassium or ammonium) phosphite, sodium (or potassium or ammonium) orthophosphite, sodium (or potassium or ammonium) hypophosphite, and phosphoric acid or salts thereof. The concentration of the phosphorus comprising material in the composition should preferably range from about 10 g/l to about 200 g/l and is preferably about 100 g/l.
One source of the phosphorus acid, orthophosphite, and/or hypophosphite is spent electroless nickel solutions. Spent electroless nickel baths may contain up to 250 grams/liter of phosphorus acid salts. The spent electroless nickel baths are normally waste treated or hauled away at some expense when the concentration of phosphorus acid salts in the baths reaches an unacceptable level. Using spent electroless nickel solutions provides a benefit to electroless nickel users by removing waste chemicals at minimal cost, as well as providing a benefit to manufacturers of the present invention by providing a raw material source at little or no cost. Preferably, the nickel ions in the spent electroless nickel solution have been removed by plating or other precipitation methods.
The conversion coating composition, optionally but preferably, also comprises a source of borate ions, fluoride ions, and/or fluoroborate ions. Most preferably, the composition comprises a source of fluoroborate ions such as sodium tetrafluoroborate or ammonium fluoroborate. Sources of borate ions include boric acid and salts thereof. Sources of fluoride include sodium fluoride, potassium fluoride, and ammonium fluoride. Preferably the concentrations of borate ions, fluoride ions, and/or fluoroborate ions in the composition ranges from about 0.1 g/l to about 200 g/l and is most preferably about 10 g/l to about 30 g/l.
The inventors have also found that it is preferably beneficial to include one or more materials selected from the group consisting of hydrofluorosilicic acid, triethanolamine, and surfactants. If used, the concentration of hydrofluorosilicic acid should preferably range from about 0.1 g/l to about 100 g/l but is most preferably from about 0.5 g/l to about 5 g/l. The inventors have found that the inclusion of triethanolamine in the conversion coating composition assists with the cleaning of the treated surfaces and therefore assists with the formation and uniformity of the conversion coating. If used, the concentration of triethanolamine in the composition should preferably range from about 1 g/l to about 100 g/l and is most preferably from about 5 g/l to about 30 g/l. Lastly, the inventors have found that the inclusion of a surfactant in the conversion coating composition is useful. Fluoro-surfactants such as Dupont FSK or 3M FC-135 surfactants are most preferred. If used, the concentration of surfactant in the composition preferably ranges from about 0.1 g/l to about 4 g/l, and is most preferably about 1 g/l.
The pH of the solution should range from about 1 to about 4, with an optimal pH of 2. The operating temperature of the solution is generally between 40xc2x0 F. and 140xc2x0 F., with a preferred temperature of between 55xc2x0 F. and 85xc2x0 F.